Deprecate Buffers.

[Vasily Chekalkin]

Hello.

Currently we have STRING and Buffers as one of core GCable objects.
Buffers were introduced long time ago, before ByteBuffer PMC. At this
point of Buffers used only in Packfiles and can be easiliy replaced
with BytBuffer.

I would like to deprecate Buffers. It will simplify life of ony GC
implementors (e.g. me with gc_massacre), cleanup code, decrease
maintenance cost, etc.

-- 
Bacek